It has become increasingly apparent over the past few years that several new technologies are emerging that will require large quantities of inexpensive alkaline polysulfide chemical reagents. One such technology has focused on the stabilization of the massive amounts of chromite ore processing residues (COPR) that have accumulated at various sites throughout the United States over the last several decades due to the absence of a cost-effective treatment methodology.
According to one application of alkaline polysulfide chemical reagents which is disclosed in U.S. Pat. No. 6,214,304 to Rosenthal, it has been found that injecting alkaline polysulfides into the flue gas of coal-fired power plants can be highly effective for enhanced removal of mercury.
Another important area of emerging technologies involves regenerative fuel cells such as described in U.S. Pat. No. 6,841,294 to Morrissey. This area of technology, which is receiving ever increasing attention, requires large amounts of polysulfide reagents that are used as electrolytes in storage batteries.
Calcium polysulfide and sodium polysulfide are generally designated as CaSx and Na2Sx, respectively, where x indicates the number of sulfur atoms. Although many different methods for making calcium polysulfide or sodium polysulfide reagents have been previously developed over a period of many years, such methods are typically directed to making one type of polysulfide or the other, but not both or multiple types simultaneously.
U.S. Pat. No. 1,434,266 to Taki is directed to a method for the manufacture of lime sulphur in which a mixture of calcium oxide (quick lime), elemental sulfur and water is steam heated above the melting point of sulfur in a pressurized revolving reactor to make a “lime sulfur” solution containing calcium polysulfide.
A similar method using a different type of reactor is disclosed in U.S. Pat. No. 4,624,419 to Hevesi et al. which is directed to an apparatus for preparing aqueous and oily sulfur containing products.
The chemical reaction for such methods is essentially:3Ca(OH)2+(2x+2)So+heat→3H2O+CaS2O3+2CaSx  (1)
Similar reactions can be written in which the hydrated lime, Ca(OH)2, is replaced with quick lime, CaO; and the calcium thiosulfate, CaS2O3, is replaced by calcium sulfite, CaSO3.
Work by others lead to the accepted conclusion that methods based on the above reaction are undesirable because: (1) they require high temperatures (near boiling) to enable part of the elemental sulfur to be reduced to the sulfide ion needed for forming polysulfide; (2) the equivalent amount of elemental sulfur that is oxidized to thiosulfate ion (or alternatively to sulfite ion) to complete the oxidation-reduction reaction causes the polysulfide yield to be low; (3) the resulting product liquor is not a mixture of polysulfides alone; and (4) the thiosulfates and sulfites generate a great deal of sludge solids that limit the concentration at which the product can be produced.
U.S. Pat. No. 1,517,522 to Volck discloses a process for producing lime sulphur compounds. Volck teaches that a preferable method for making calcium polysulfide involves adding hydrogen sulfide to the lime-sulfur reaction to eliminate the consumption of elemental sulfur needed for internal generation of the sulfide. The reaction for calcium pentasulfide occurs as follows:Ca(OH)2+H2S+4So→CaS5+2H2O  (2)
Volck points out that the advantages of the above reaction include: (1) only polysulfide is produced; (2) the reaction proceeds quickly at ambient or warmer temperatures; (3) less sludge solids are produced; and (4) higher concentrations of polysulfide can be achieved. A related method for making calcium polysulfide using lime and hydrogen sulfide was subsequently developed and disclosed in U.S. Pat. No. 2,135,879 to Shiffler et al.
Many of the methods for making sodium polysulfide were developed specifically for applications in the pulp and paper industry where alkaline polysulfide cooking liquors are used to digest wood. U.S. Pat. No. 3,216,887 to Landmark and U.S. Pat. No. 4,024,229 to Smith et al. disclose oxygenation of sodium sulfide to form sodium polysulfide and sodium thiosulfate. U.S. Pat. No. 5,234,546 to Chamblee discloses a variety of techniques for recovering sodium polysulfide from various cooking liquors using techniques such as fuel cells and activated carbon catalyzed oxidation reactions. U.S. Pat. No. 6,264,819 to Andoh et al. discloses an electrolytic oxidation method for producing a sodium polysulfide cooking liquor which involves electrolytically oxidizing pulping liquors. Such applications are generally limited to relative dilute solutions of sodium polysulfide, e.g., typically in the range of 0.5% to 8.0%. Moreover they generally produce, or are susceptible to producing, thiosulfate along with the polysulfide.